This invention is generally directed to improved photoresponsive devices, and more specifically the present invention is directed to an improved photoresponsive imaging member containing as an overcoating layer arylmethanes, such as bis-(4-diethylamino-2-methylphenyl)-phenylmethane. The improved photoresponsive imaging member of the present invention, which also contains a photogenerating layer, and charge transport layer, is useful in electrostatographic imaging systems, including xerographic imaging systems, wherein latent electrostatic latent images are formed on the member, followed by development of the image, and subsequent transfer to a suitable substrate.
A number of photoresponsive imaging members, which are useful in electrostatic imaging systems are known. One such member is comprised of a conductive substrate containing on its surface a layer of photoconductive insulating material, such as amorphous selenium. In some instances, there can be situated between the substrate and the amorphous selenium a thin barrier layer of aluminum oxide, for the purpose of preventing charge injection from the substrate into the selenium upon charging of the plate surface. Other inorganic photoresponsive materials are known including, for example, alloys of selenium, such as arsenic selenium, selenium arsenic tellurium, and mixtures of selenium with other substances. Also recently, there has been disclosed organic photosensitive materials, wherein the charge carrier generating function, and the charge carrier transport function, are accomplished by discrete contiguous layers. Moreover, photoreceptors are known which include an overcoating layer of an electrically insulating polymeric material, and in conjunction with this overcoated type photoreceptor, there have been proposed a number of imaging methods. However, the art of xerography continues to advance and more stringent demands need to be met by the copying apparatus in order that performance standards may be increased primarily for the purpose of obtaining higher quality images. Also, there are needed protective layers for photoresponsive devices in order to prevent these devices from degrading under cyclic conditions.
There is disclosed in U.S. Pat. No. 3,041,167 an electrophotographic imaging member comprised of a conductive substrate, a photoconductive insulating layer, and an overcoating layer of an electrically insulating polymeric material. This member can be selected for use in electrophotographic imaging methods where the member is, initially charged with electrostatic charge of a first polarity, and imagewise exposed to form an electrostatic latent image, which can then be transferred to form a visible image after development.
Disclosed in U.S. Pat. No. 4,265,990 are organic layered photoresponsive devices comprised of a substrate, a generating layer, and a transport layer. Examples of generating layers include inorganic photoconductive materials such as triganol selenium, and organic photoconductive materials such as metal phthalocyanines, metal free phthalocyanines, and vanadyl phthalocyanine, while examples of carrier transport layers include certain diamines dispersed in a resin binder, the diamines being of the formula as shown, for example, in the '990 patent. Additionally disclosed in U.S. Pat. No. 4,059,935 a photosensitive member having at least two electrically operative layers, the first layer containing trigonal selenium and the second layer containing a contiguous charge transport layer comprised of a transparent electrically inactive organic resinous material containing from about 15 to about 75 percent by weight of a bis-(4-diethylamino-2-methylphenyl)-phenylmethane. The use of the bis-(4-diethylamino-2-methylphenyl)-phenylmethane compound as an overcoating for a photoresponsive device is, however, not disclosed in this patent.
Although the photoresponsive devices described in the prior art are effective for their intended purposes, there continues to be a need for, improved photoresponsive devices. Further, some of the prior art photoresponsive imaging devices, particularly those comprised of transport and generating layers, may suffer deficiencies during the cleaning processing sequence. In this sequence, it is believed that the charge carrier transport layers containing the diamines of the '990 patent may be susceptible to unpredictable positive charge acceptance, from a positive charging precleaning corotron device. This results primarily from the degradation of the top layer portions of the diamine transport layer, which in turn causes positive charges to be injected from the top surface of the device into other layers, resulting in low and unpredictable positive charge acceptance for this device. Accordingly, this unpredictable positive charge acceptance following cleaning would cause variations in negative charges subsequently applied to the device, causing fluctuations in image densities. Another problem encountered with the degradation of the top layer portions of the diamine transport layer relate to loss of image resolution, since the diamines after degradation from corona effluents and the like become conductive. While it is not believed that the surface of the triphenylmethane transport layers of the '935 patent become conductive because of degradation, the bulk of these layers are susceptible to charge trapping caused by degradation, which trapping causes a residual potential to form after each charge exposure step in the image cycle. This accumulation of residual potential on the photoresponsive device causes an increase in the background density of the resulting images.